Question
For the gas phase, reaction,
PCl5 (g) ⇌ PCl3 (g) Cl2 (g)
Which of the following conditions are correct?
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Δ H = 0 and ΔS > 0
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ΔH >0 and ΔS > 0
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Δ H < 0 and ΔS < 0
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ΔH > 0 and ΔS < 0
Solution
B.
ΔH >0 and ΔS > 0
ΔH = ΔE + ΔnRT
Δn = number of moles of product - number of moles of reactants
ΔG = ΔH - TΔS
For a spontaneous process, ΔG must be negative
PCl5 (g) ⇌ PCl3 (g) Cl2 (g)
In this reaction
Δn = 2-1 = 1
Thus, ΔH is positive, ie, >0
If ΔH is positive, then to maintain the value of ΔG negative, ΔS should be positive , ie, ΔS>0.
